(a) Field of the Invention
The present invention relates to a lens system using inhomogeneous material, especially a graded refractive index (GRIN) single lens system used as an objective lens system for optical video disks, etc.
(b) Description of the Prior Art
Recently there have developed apparatuses which read, by the converging of a laser beam to a microspot, the information which is recorded with high density on recording medium such as optical video disks, digital audio disks, etc.
In such apparatuses, it is necessary for an objective lens system used for the recording and the playing back of information to be compact and light because the objective lens system is driven directly for the purpose of autofocusing and auto-tracking. It is also necessary for the objective lens system to have a large N.A. in order to obtain a smaller spot size of a laser beam which is converged on a recording medium.
As such an objective lens system, a combination of a plurality of homogeneous spherical lenses or a single homogeneous aspherical lens, especially for the purposes of being compact and light, has hitherto been in use.
Moreover, besides these homogeneous lenses, a GRIN single lens system using inhomogeneous material for economy of manufacture, compactness, and light weight has been known recently.
In the early GRIN lens system, only the correction of spherical aberration was considered.
As is well known, it is necessary for an objective lens system used for optical video disks, etc. to have aberrations well-corrected in the range of diameters of 0.1-0.2 mm on the disk surface and, therefore, not only spherical aberration but also coma should be well-corrected.
There have been known the GRIN single lens systems the surface of which are suitably spherical, and the higher order coefficients of the refractive index distribution are arranged so that not only spherical aberration but also coma (sign condition) is corrected. They are, for example, disclosed in Japanese Published Unexamined Patent Application Nos. 122512/83 and 62815/84. In the former of these prior art references, the refractive index distribution is expressed by EQU n=n.sub.00 +n.sub.10 r.sup.2 +n.sub.20 r.sup.4,
and both radii of curvatures of two refracting surfaces and the higher order coefficients n.sub.10, n.sub.20 of the refractive index distribution are arranged so that the coefficient of third order aberration becomes almost zero. In the latter thereof, the refractive index distribution is expressed by EQU n.sup.2 =n.sub.0.sup.2 [1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 ],
and both the radii of curvatures of two surfaces and the higher order coefficients h.sub.4, h.sub.6 are arranged so that spherical aberration and sign condition are corrected to be almost zero.
In the former of these two prior art references, the correction of aberrations does not reach the level of practical use, but in the latter thereof, aberrations are well-corrected to the level of practical use in view of aberrations.
However, in all the prior art, N.A. is 0.45 at most, which does not satisify the requirement of the N.A. being larger.